The invention relates to a switched-mode frame-scan control circuit used in particular in television receivers.
The vertical movement of the spot on the screen of a cathode-ray tube is provided by coils, called vertical-deflection or frame-scan coils, through which flows a current generally in saw-tooth form. In the case of the French standard, this current has a period of 20 milliseconds and a frame scan return time less than 1 ms.
In the prior art, this current is generated by means of electronic devices comprising generally a disabled oscillator supplying a saw-tooth voltage synchronized with the frame frequency, a connecting stage with high input impedance and low output impedance, and an output stage supplying to the vertical deflection coils the current having the amplitude and shape required for vertical deflection of the spot of the cathode-ray tube. This output stage may be a class A biased power amplifier, or else a complementary-symmetry or class AB biased "push-pull" amplifier.
Chopper devices, provided with active switches such as thyristors, are also used for generating the frame-scan current.
In this case, one of the active switches ensures the vertical scan of the upper half of the screen, by charging a capacitor connected in parallel with the vertical deflection winding by means of voltage pulses of decreasing width in time and of a given polarity.
The second switch ensures the vertical scan of the lower half of the screen, by charging the condenser by means of voltage pulses of decreasing width in time and of a polarity opposite the preceding one.
The discharge of this condenser causes in fact a saw-tooth current to flow through the vertical deflection winding.
The two paths of these chopper devices are monodirectional in current flow, each in a different direction.
The French patent application filed under the No. 78/22266 on July 27, 1978, in the name of the applicant proposes a single-path chopper device, i.e. a single active switch.
In fact, for this the vertical deflector is connected in series with a measuring resistance and a connecting capacitor whose value is sufficiently high for the voltage at the terminals of the unit thus formed to keep the same polarity whatever the direction of the current flowing through the vertical deflector. This unit then behaves sometimes as generator, sometimes as a receiver, and is connected to a circuit for generating a saw-tooth signal from the line-scan return signal, monodirectional in voltage and bidirectional in current, controlled by a control circuit synchronized to the line frequency.
In accordance with the previously mentioned French patent application No. 78/22266, the active switch is enabled during the outward line scan, the time when it starts to conduct defining the current flowing through the deflector.